Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs, fuel economy and emissions. Turbochargers may have a variable geometry turbine (VGT) wherein the impeller blades (or vanes) of the turbine are regulated to vary boost pressure and exhaust pressure. A position of the impeller blades of the VGT may be varied based on a plurality of factors including engine speed, torque demand, desired response time, fuel economy, intake and exhaust manifold pressure, and emissions requirements. By varying an aspect ratio of the turbocharger, the VGT adjustment enables pumping losses to be reduced.
One example approach for adjusting VGT geometry is shown by Buckland et al. in U.S. Pat. No. 6,672,060. Therein, VGT geometry is adjusted via a feedback control loop taking into account a difference between an actual intake manifold pressure and a desired intake manifold pressure.
However, the inventors have recognized potential issues with the above approach. As one example, it may not be possible to sufficiently optimize pumping work and the occurrence of exhaust pressure spikes. In particular, during transient and off idle engine operation, for example during tip-in and tip-out events, there may be exhaust pressure spikes caused by rapid changes in the flow of gases into the exhaust manifold without a corresponding change in the flow of gases out of the exhaust manifold resulting in an increase in the exhaust manifold pressure. Accordingly an increase in the pressure difference between the exhaust and intake manifold may be observed. Typically pressure control in the intake and exhaust manifold is performed based on the intake manifold pressure which reacts to changes and disturbances more slowly than the exhaust manifold pressure (due to larger intake manifold volume compared to smaller exhaust manifold volume and because disturbances such as fueling changes first impact the exhaust manifold and only afterwards reach the intake manifold). Under such circumstances, the VGT (or exhaust gas recirculation) actuator is not adjusted to increase the exhaust manifold outflow until the pressure increase is observed in the intake pressure, during which time the exhaust pressure may have rapidly increased to undesirable levels. During this time, high exhaust manifold pressure and exhaust pressure spikes result in an increase in the delta pressure across an engine, increasing engine pumping work which may adversely affect engine efficiency, performance, emissions, and fuel economy. Also during this time, the expansion ratios of the turbine may increase excessively which may cause damage to the turbocharger hardware. In addition, such exhaust pressure spikes and high expansion ratios may lead to high cycle fatigue and eventually degradation of several engine components, such as seals, gaskets, exhaust valves and cylinder components.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method for exhaust pressure control includes a method for a boosted engine system comprising: adjusting a variable geometry turbine (VGT) based on each of an engine speed, an exhaust pressure, and a difference between the exhaust pressure and an intake pressure to maintain a desired delta pressure and boost pressure. The inventors herein have recognized that by monitoring the pressure difference (delta pressure) between an exhaust and an intake manifold and adjusting the VGT geometry based on the pressure difference, VGT adjustments can be scheduled more effectively to reduce the occurrence of high delta pressures across the engine and subsequently high exhaust pressure and exhaust pressure spikes. In addition, EGR can be leveraged to reduce the pumping work by adjusting the opening of the EGR valve in order to increase EGR flow from the exhaust manifold to the intake manifold, thereby reducing exhaust manifold pressure and engine pumping work.
In one example, during transient engine operations, at least one of VGT vane actuator, wastegate valve, and EGR valve may be adjusted to control the pressure difference across an engine, thereby controlling/reducing engine pumping work, exhaust pressure spikes, and excessive expansion ratios. As an example, the position of a VGT vane, wastegate valve opening, and/or an EGR valve opening may be continuously adjusted based on actual pressure difference between an exhaust and an intake manifold to reduce a pressure difference across the engine between the intake and exhaust pressure. In particular, in addition to an existing proportional-integral (PI) controller (for example control based on boost pressure error from a desired boost pressure or exhaust pressure error from a desired exhaust pressure and other signals), a proportional-derivative (PD) controller (control based on a difference between exhaust and intake manifold pressure) may be used to adjust at least one of VGT vanes, EGR valve, and wastegate valve opening in order to maintain an optimal pressure difference (or reduce excessive pressure differences) between the exhaust and intake manifold. The PD controller may receive signals including pressure difference between exhaust and intake manifold, intake and exhaust manifold pressure, flow and engine speed from the respective sensors and these signals may be utilized for adjusting the VGT vane actuator, the wastegate valve, and/or the EGR valve position. In one example, gains may increase based on an increase in engine speed and/or an increase in exhaust pressure, resulting in an increase (both magnitude and rate of increase) in the opening of the VGT vanes, wastegate valve, and/or the EGR valve. Similarly, gains may decrease based on a decrease in engine speed and/or a decrease in exhaust pressure, resulting in a decrease in the opening of the VGT vanes, wastegate valve, and/or the EGR valve. As the VGT is coupled to the exhaust manifold, by increasing the opening of the VGT vanes via actuation of a VGT actuator it is possible to vary the VGT aspect ratio and thereby reduce exhaust pressure and spikes in exhaust pressure with little effect on intake manifold pressure. Likewise, as the EGR valve opening is located at the exhaust manifold, exhaust pressure may be effectively reduced by increasing EGR valve opening. Similarly, by routing exhaust via a wastegate passage (the opening of which is controlled by a wastegate valve), exhaust pressure spikes may be decreased. In alternate examples, a ratio between exhaust and intake pressure may be used by the control system to adjust the VGT and EGR openings simultaneously.
In this way, VGT geometry (vanes), wastegate valve position, and/or EGR valve position may be effectively adjusted via their respective actuators in order to reduce the difference between the exhaust manifold pressure and intake manifold pressure of a boosted engine. By adjusting based on engine speed, pressure difference between the exhaust and intake manifold (delta pressure), and exhaust pressure so as to control the difference between the exhaust and intake manifold pressure, engine pumping losses may be optimized. In addition, exhaust pressure spikes and excessive expansion ratios may be avoided thereby enhancing engine performance and fuel efficiency. By varying the gains as a function of engine speed and exhaust pressure, along with the difference between an exhaust and intake manifold as the control input, the controller may more aggressively control the VGT vanes, wastegate valve position, and EGR valve positions as the pressure difference increases or decreases. In one example, if the pressure difference is small, the controller may provide minor adjustments to the VGT and EGR actuators. In another example, if the pressure difference increases beyond a threshold, the controller may aggressively adjust the actuators in order to reduce the pressure difference, such as with a higher gain tuning. The technical effect of controlling delta pressure across the engine is reduced engine pumping losses, reduced/decreased exhaust pressure spikes, and reduced excessive expansion ratios is that damage to turbocharger and other hardware components due to fatigue may be reduced and further, emissions, performance, and fuel economy may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.